The present invention relates to a process for the sequential control of a liquid crystal matrix display means having different optical responses in alternating and steady electric fields. It is used in optoelectronics in the production of the liquid crystal displays used as converters of electrical informations into optical informations and for the binary display of complex images or alphanumeric characters.
More specifically, the invention relates to the sequential control of a matrix display means incorporating a display cell containing a ferroelectric liquid crystal and having negative dielectric anisotropy, while having different optical responses for a.c. and d.c. exciting signals. Hitherto this type of liquid crystal is the only one having different optical responses in alternating and steady fields.
Such liquid crystals are generally obtained by mixing a ferroelectric chiral smectic liquid crystal C and a smectic or cholesteric nematic liquid crystal A having a negative dielectric anisotropy.
FIG. 1 shows in longitudinal section a display cell containing such a liquid crystal. This display cell 10 is formed from two transparent insulating walls 12, 14, which are generally made from glass. These parallel walls are joined at their edges by means of a weld 13 serving as a sealing joint.
Display cell 10 contains a mixture of liquid crystals 16 containing a ferroelectric chiral smectic liquid crystal C and a nematic liquid crystal with negative dielectric anisotropy. A nematic liquid crystal with negative dielectric anisotropy is generally obtained by grafting in the core of the molecules of the nematic liquid crystal an electronegative group, e.g. a halogen, such as chlorine.
The inner face of wall 12 of cell 10 is covered with m parallel conductive strips 18 serving as the row electrodes. In the same way, the inner face of the cell wall 14 is covered with n parallel conductive strips 20 serving as the column electrodes. As the row and column electrodes intersect, each intersection defines an elementary zone of the liquid crystal, whose electrooptical property can be selectively excited. The different elementary display zones are distributed in matrix form. These row and column electrodes 18, 20 are connected to an electric power supply 8, so that an electric field can be applied to one or more liquid crystal zones.
FIG. 2 shows the structure of the molecules of the liquid crystal mixture 16. Molecules 22 are those of the ferroelectric chiral smectic liquid crystal C and molecules 24 are those of the nematic liquid crystal with negative dielectric anisotropy.
The molecules 22 are elongated and arranged in parallel layers. Molecules 22 have the same orientation n in the same layer. The longitudinal axis of the molecules 22 of the same layer 26 is inclined by an angle .theta. with respect to the normal to layers 26, designated D. Each molecule 22 has an electric dipole p perpendicular to direction n of molecules 22 and parallel to layers 26. The molecular direction n and the dipole p precess about the normal D from one layer 26 to the other.
Molecules 24 are also elongated and their molecular orientation and layer-form distribution are imposed by those of the molecules 22. Therefore molecules 24 are parallel to molecules 22 in the same layer. Each molecule 24 has an electric dipole i perpendicular to molecular direction n.
FIG. 3 shows the two possible orientations of the molecules of liquid crystal mixture 16. With reference to FIG. 3, an explanation will now be given of the behaviour of molecules 22 and 24 of mixture 16 in the presence of an electric field applied thereto.
The two possible orientations A and B are defined with respect to the normal of layers D. These two orientations A and B are in a longitudinal plane .pi. parallel to the plane of the two walls 12, 14 of the display cell. In the first orientation A, molecules 22 and 24 are inclined by an angle +.theta. with respect to direction D and the electric dipole p is oriented from bottom to top in FIG. 3.
In the second orientation B, molecules 22 and 24 are inclined by an angle -.theta. relative to directoion D and the electric dipole p is oriented from top to bottom in FIG. 3.
When an alternating electric field E.sub.S is produced between electrodes 18 and 20 of display cell 10 containing mixture 16, molecules 22 and 24 are subject to a torque or moment .GAMMA..sub.S tending to align the dipoles of the molecules with the alternating field E.sub.S. Torque .GAMMA..sub.S is a restoring torque. The prior orientation A or B of molecules 22 and 24 is retained. Dipole i serves as a stabilizer by aligning parallel with said field E.sub.S.
When a steady magnetic field E.sub.C is produced between electrodes 18 and 20 of the display cell 10 containing liquid crystal 16, the dipoles of molecules 22 and 24 are subject to a moment or torque .GAMMA..sub.C tending to align molecules 22 and 24 with the steady field E.sub.C. This torque .GAMMA..sub.C is a tilting moment. Molecules 22, 24, previously oriented either in accordance with A or B are oriented according to the same direction A or B. The orientation obtained is that for which the electric dipole p is oriented parallel to field E.sub.C and in the same sense as the latter. Thus, dipole p serves as a destabilizer.
Numerous processes for the sequential control of a liquid crystal matrix display means are known, like those described hereinbefore using a.c. or d.c. exciting signals for locally controlling the electrooptical property of said liquid crystals. However, these processes unfortunately require m+n control circuits or connections for displaying a matrix of m.times.n elementary display zones defined by the intersection of m row electrodes and n column electrodes. Moreover, the use of a direct current progressively deteriorates the liquid crystal.
The present invention relates to process for the sequential control of a liquid crystal matrix display means only requiring four control circuits and connections for the display of a random number of elementary display zones. This process is based on the use of an in particular ferroelectric liquid crystal with negative dielectric anisotropy having different optical responses for the a.c. and d.c. exciting signals.